Edge coupler

ABSTRACT

A composite optical waveguide is constructed using an array of waveguide cores, in which one core is tapered to a larger dimension, so that all the cores are used as a composite input port, and the one larger core is used as an output port. In addition, transverse couplers can be fabricated in a similar fashion. The waveguide cores are preferably made of SiN. In some cases, a layer of SiN which is provided as an etch stop is used as at least one of the waveguide cores. The waveguide cores can be spaced away from a semiconductor layer so as to minimize loses.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority to, U.S.patent application Ser. No. 14/798,780, filed Jul. 14, 2015, nowallowed, which claims priority to U.S. Provisional Application No.62/170,772, filed Jun. 4, 2015, each of which is hereby incorporated byreference herein in its entirety.

FIELD OF THE INVENTION

The invention relates to optical couplers in general and particularly tooptical couplers that optically connect optical fibers and chips.

BACKGROUND OF THE INVENTION

Because silicon has a high refractive index, a single mode siliconwaveguide exhibits sub-micron mode size. The observed loss is high whenan optical mode couples to a single mode fiber because of a large modemismatch. Grating couplers have large mode profiles, but their bandwidthis limited and they are usually polarization dependent. Edge couplershave broad bandwidth with small polarization dependent loss. Howevertraditional inverted-taper-based edge couplers usually are constructedto work with lensed fiber because their mode sizes are usually around 3μm. This imposes difficulties in packaging, because it requires accuratealignment between the edge coupler and the lensed fiber. To match to 10μm mode field diameter (MFD), people have built SiO₂ waveguides withSi/SiN waveguide inside the SiO₂, but this method requires a siliconundercut with wet etch to avoid substrate loss which is costly, riskyand time-consuming.

Also known in the prior art is Assefa et al., U.S. Pat. No. 7,738,753,issued Jun. 15, 2010, which is said to disclose an optoelectroniccircuit fabrication method and integrated circuit apparatus fabricatedtherewith. Integrated circuits are fabricated with an integral opticalcoupling transition to efficiently couple optical energy from an opticalfiber to an integrated optical waveguide on the integrated circuit.Layers of specific materials are deposited onto a semiconductor circuitto support etching of a trench to receive an optical coupler thatperforms proper impedance matching between an optical fiber and anon-circuit optical waveguide that extends part way into the transitionchannel. A silicon based dielectric that includes at least a portionwith a refractive index substantially equal to a section of the opticalfiber is deposited into the etched trench to create the optical coupler.Silicon based dielectrics with graded indices are also able to be used.Chemical mechanical polishing is used finalize preparation of theoptical transition and integrated circuit.

Other relevant publicly available documents that describe the prior artinclude: M. Wood, P. Sun, and R. M. Reano, “Compact cantilever couplersfor low-loss fiber coupling to silicon photonic integrated circuits,”Opt. Express, vol. 20, no. 1, p. 164, 2012; L. Chen, C. R. Doerr, Y.Chen, and T. Liow, “Low-Loss and Broadband Cantilever Couplers BetweenStandard Cleaved Fibers and High-Index-Contrast Si3N4 or Si Waveguides,”IEEE Photonics Technol. Lett., vol. 22, no. 23, pp. 1744-1746, 2010; L.Jia, J. Song, T.-Y. Liow, X. Luo, X. Tu, Q. Fang, S.-C. Koh, M. Yu, andG. Lo, “Mode size converter between high-index-contrast waveguide andcleaved single mode fiber using SiON as intermediate material,” Opt.Express, vol. 22, no. 19, p. 23652, September 2014; and R. Takei, E.Omoda, M. Suzuki, S. Manako, T. Kamei, M. Mori, and Y. Sakakibara,“Low-loss optical interlayer transfer for three-dimensional opticalinterconnect,” in Proceedings of 10th International Conference on GroupIV Photonics (Seoul, South Korea, 2013), pp. 91-92.

There is a need for improved couplers for optically interconnectingchips and optical fibers.

SUMMARY OF THE INVENTION

According to one aspect, the invention features a composite waveguide,comprising: a first group of waveguide cores on a substrate, the firstgroup of waveguide cores having an optical propagation direction, eachof the first group of waveguide cores having at a respective first endthereof a first cross section defined by two dimensions measured alongorthogonal coordinates and at a respective second end thereof a secondcross section defined by two dimensions measured along orthogonalcoordinates; the first group of waveguide cores having an input portcomprising at least one of: the respective first ends togethercomprising the optical input port for directed illumination, and asegment of one or more of the first group of waveguide cores comprisingthe optical input port for evanescent waves; and the first group ofwaveguide cores having an output port comprising at least one of: therespective second ends of the one or more of the first group ofwaveguide cores comprising the output port for directed illumination;and a segment of one or more of the first group of waveguide corescomprising the optical output port for evanescent waves.

In one embodiment, the substrate is a semiconductor chip.

In another embodiment, one or more of the first group of waveguide coreshas a different second cross section than the second cross section ofeach of the remaining ones of the first group of waveguide cores.

In a further embodiment, the different second cross section is a largersecond cross section than the second cross section of each of theremaining ones of the first group of waveguide cores.

In one more embodiment, the different second cross section is a smallersecond cross section than the second cross section of each of theremaining ones of the first group of waveguide cores.

In one embodiment, the composite waveguide is configured to operatebidirectionally.

In another embodiment, at least one of the first end and the second endof the composite waveguide is configured as a butt coupler.

In a further embodiment, at least one of the first end and the secondend of the composite waveguide is configured as a taper coupler.

In still another embodiment, at least one of the first end and thesecond end of the composite waveguide is configured as an evanescentcoupler.

In one embodiment, all of the first group of waveguide cores havesubstantially equal first cross sections.

In another embodiment, the at least two of the first group of waveguidecores have unequal first cross sections.

In yet another embodiment, the first group of waveguide cores aredisposed in a one-dimensional array.

In still another embodiment, the first group of waveguide cores aredisposed in a two-dimensional array.

In a further embodiment, the first group of waveguide cores comprise amaterial selected from the group of materials consisting of crystallinesilicon, poly-silicon, amorphous silicon, silicon nitride, siliconoxynitride, silicon dioxide, doped silicon dioxide and a polymer.

In yet a further embodiment, the first cross section defined by twodimensions measured along orthogonal coordinates has a first dimensionand a second dimension each no larger than 1 micron.

In an additional embodiment, the respective first end of each of thefirst group of waveguide cores is located at a facet.

In one more embodiment, the facet is a facet selected from the group offacets consisting of etched facets, polished facets, sawed facets,angled facets and curved facets.

In still a further embodiment, the respective first end of each of thefirst group of waveguide cores is located within 50 microns of a facet.

In one embodiment, the composite waveguide further comprising: a secondgroup of waveguide cores having a second optical propagation direction,the second group of waveguide cores disposed on the substrate, thesecond group of waveguide cores displaced from the first group ofwaveguide cores in a direction transverse to the optical propagationdirection of the first group of waveguide cores, the second group ofwaveguide cores in optical communication with the first group ofwaveguide cores; each of the second group of waveguide cores having at arespective first end thereof a first cross section defined by twodimensions measured along orthogonal coordinates and at a respectivesecond end thereof a second cross section defined by two dimensionsmeasured along orthogonal coordinates; the second group of waveguidecores having an input port comprising at least one of: the respectivefirst ends together comprising the optical input port for directedillumination, and a segment of one or more of the second group ofwaveguide cores comprising the optical input port for evanescent waves;and the second group of waveguide cores having an output port comprisingat least one of: the respective second ends comprising the output portfor directed illumination, and a segment of one or more of the secondgroup of waveguide cores comprising the optical output port forevanescent waves.

In another embodiment, one or more of the second group of waveguidecores has a different second cross section than the second cross sectionof each of the remaining ones of the second group of waveguide cores.

In a further embodiment, the different second cross section is a largersecond cross section than the second cross section of each of theremaining ones of the second group of waveguide cores.

In one more embodiment, the different second cross section is a smallersecond cross section than the second cross section of each of theremaining ones of the second group of waveguide cores.

In one embodiment, the composite waveguide is configured to operatebidirectionally.

In another embodiment, at least one of the first end and the second endof the composite waveguide is configured as a butt coupler.

In a further embodiment, at least one of the first end and the secondend of the composite waveguide is configured as a taper coupler.

In still another embodiment, at least one of the first end and thesecond end of the composite waveguide is configured as an evanescentcoupler.

In a further embodiment, the second group of waveguide cores isconfigured to provide an optical signal at a location displaced in thetransverse direction from the optical input port of the first group ofwaveguide cores.

In another embodiment, the second group of waveguide cores comprise amaterial selected from the group of materials consisting of crystallinesilicon, poly-silicon, amorphous silicon, silicon nitride, siliconoxynitride, silicon dioxide, doped silicon dioxide and a polymer.

In yet another embodiment, the first cross section defined by twodimensions measured along orthogonal coordinates has a first dimensionand a second dimension each no larger than 1 micron.

In still another embodiment, the respective first end of each of thesecond group of waveguide cores is located at a facet.

In a further embodiment, the respective first end of each of the secondgroup of waveguide cores is located at a facet.

In yet a further embodiment, the facet is a facet selected from thegroup of facets consisting of etched facets, polished facets, sawedfacets, angled facets and curved facets.

In an additional embodiment, the respective first end of each of thefirst group of waveguide cores is located within 50 microns of a facet.

In still a further embodiment, the second optical propagation directionis parallel to the optical propagation direction of the first group ofwaveguide cores.

According to another aspect, the invention relates to a method ofmanufacturing a composite waveguide. The method comprises the steps of:providing a device fabricated with etch stop layers; using at least oneof the etch stop layers as a waveguide core of a composite waveguidehaving an optical input port and an optical output port.

In one embodiment, the etch stop layers comprise a silicon nitride etchstop layer.

In one embodiment, the composite waveguide comprises a group ofwaveguide cores, each of the group of waveguide cores having at arespective first end thereof a first cross section defined by twodimensions measured along orthogonal coordinates and having at arespective second end thereof a second cross section defined by twodimensions measured along orthogonal coordinates; the first group ofwaveguide cores having an input port comprising at least one of: therespective first ends together comprising the optical input port fordirected illumination, and a segment of one or more of the first groupof waveguide cores comprising the optical input port for evanescentwaves; and the first group of waveguide cores having an output portcomprising at least one of: the respective second ends comprising theoutput port for directed illumination; and a segment of one or more ofthe first group of waveguide cores comprising the optical output portfor evanescent waves.

According to a further aspect, the invention relates to a method ofusing a composite waveguide. The method comprises the steps of:providing a composite waveguide having an optical input port and anoptical output port, comprising: a first group of waveguide cores on asubstrate, the first group of waveguide cores having an opticalpropagation direction, each of the first group of waveguide cores havingat a respective first end thereof a first cross section defined by twodimensions measured along orthogonal coordinates and having at arespective second end thereof a second cross section defined by twodimensions measured along orthogonal coordinates; the first group ofwaveguide cores having an input port comprising at least one of: therespective first ends together comprising the optical input port fordirected illumination, and a segment of one or more of the first groupof waveguide cores comprising the optical input port for evanescentwaves; and the first group of waveguide cores having an output portcomprising at least one of: the respective second ends comprising theoutput port for directed illumination; and a segment of one or more ofthe first group of waveguide cores comprising the optical output portfor evanescent waves; causing optical illumination to impinge on theoptical input port of the composite waveguide; and recoveringtransmitted optical illumination from the optical output port of thecomposite waveguide.

The foregoing and other objects, aspects, features, and advantages ofthe invention will become more apparent from the following descriptionand from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood withreference to the drawings described below, and the claims. The drawingsare not necessarily to scale, emphasis instead generally being placedupon illustrating the principles of the invention. In the drawings, likenumerals are used to indicate like parts throughout the various views.

FIG. 1 is a cross section diagram of a composite waveguide according toprinciples of the invention.

FIG. 2A is a perspective diagram of a composite waveguide having a groupof waveguides according to the principles of the invention.

FIG. 2B is another perspective diagram of a composite waveguide having agroup of waveguides according to the principles of the invention.

FIG. 2C is a side view diagram showing the optical intensity in thecomposite waveguide of FIG. 2B as a function of distance from the inputfacet.

FIG. 2D is a schematic diagram of tapered waveguide cores seen in a viewfrom the top.

FIG. 2E is a schematic diagram of tapered waveguide cores seen in a viewfrom the side.

FIG. 3A-FIG. 3D are diagrams of mode profiles of the group of waveguidecores for selected widths at the edge coupler.

FIG. 4A is a diagram of a mode profile of a composite waveguide of theinvention having two waveguide cores.

FIG. 4B is a diagram of a mode profile of a composite waveguide of theinvention having three waveguide cores.

FIG. 4C is a diagram of a mode profile of a composite waveguide of theinvention having four waveguide cores.

FIG. 5A is a schematic diagram of the propagation of directedillumination along a waveguide.

FIG. 5B is a schematic diagram of the propagation of evanescentradiation between two waveguides.

FIG. 6 is a diagram in cross section of a semiconductor chip thatincludes composite waveguides constructed according to principles of theinvention.

FIG. 7 is a diagram in cross section of another view of thesemiconductor chip of FIG. 6.

FIG. 8 is a diagram in cross section of another view of thesemiconductor chip of FIG. 6.

DETAILED DESCRIPTION Acronyms

A list of acronyms and their usual meanings in the present document(unless otherwise explicitly stated to denote a different thing) arepresented below.

AMR Adabatic Micro-Ring

APD Avalanche Photodetector

ARM Anti-Reflection Microstructure

ASE Amplified Spontaneous Emission

BER Bit Error Rate

BOX Buried Oxide

CMOS Complementary Metal-Oxide-Semiconductor

CMP Chemical-Mechanical Planarization

DBR Distributed Bragg Reflector

DC (optics) Directional Coupler

DC (electronics) Direct Current

DCA Digital Communication Analyzer

DRC Design Rule Checking

DUT Device Under Test

ECL External Cavity Laser

FDTD Finite Difference Time Domain

FOM Figure of Merit

FSR Free Spectral Range

FWHM Full Width at Half Maximum

GaAs Gallium Arsenide

InP Indium Phosphide

LiNO₃ Lithium Niobate

LIV Light intensity (L)-Current (I)-Voltage (V)

MFD Mode Field Diameter

MPW Multi Project Wafer

NRZ Non-Return to Zero

PIC Photonic Integrated Circuits

PRBS Pseudo Random Bit Sequence

PDFA Praseodymium-Doped-Fiber-Amplifier

PSO Particle Swarm Optimization

Q Quality factor

$Q = {{2\;\pi \times \frac{{Energy}\mspace{14mu}{Stored}}{{Energy}\mspace{14mu}{dissipated}\mspace{14mu}{per}\mspace{14mu}{cycle}}} = {2\;\pi\; f_{r} \times {\frac{{Energy}\mspace{14mu}{Stored}}{{Power}\mspace{14mu}{Loss}}.}}}$

QD Quantum Dot

RSOA Reflective Semiconductor Optical Amplifier

SOI Silicon on Insulator

SEM Scanning Electron Microscope

SMF Single Mode Fiber

SMSR Single-Mode Suppression Ratio

TEC Thermal Electric Cooler

WDM Wavelength Division Multiplexing

An edge coupler is described that can be integrated into a fabricationline operating a CMOS process. An edge coupler fabricated according toprinciples of the invention provides a high coupling efficiency and alow polarization dependent loss when coupling to a standard single modefiber (SMF).

The edge coupler can be any of a butt coupler, a coupler that canevanescently couple one waveguide to another or a coupler that uses ataper to couple into a structure such as a PIC chip.

As used herein the terms “a group of waveguides” or “a group ofwaveguide cores” means one or more waveguides that function together asa group. Mathematically one can define a group or a set as having noelements at all (the null group or null set), one element (the identityelement), or more than one element. However, here we are explicitlyexcluding the null group from consideration, because the null group doesnot conform to any physical reality, while the other groups can and docorrespond to physical embodiments.

We describe an edge coupler that is provided as a composite waveguidewhich comprises multiple waveguide cores. In various embodiments, thegroup of waveguide cores is fabricated using one or more silicon nitridelayers that are used to stop the oxide etch in a typical damasceneprocess. The process by which the composite waveguide is fabricated canbe seamlessly integrated into a CMOS fabrication process. The compositewaveguide can be fabricated using a standard CMOS fabrication line. Oneor more SiN strips can be configured to match the mode on the chip to astandard 10 μm single mode fiber for low mode mismatch loss. In atypical SOI wafer, the thickness of the buried oxide (BOX) layer is 2μm. As the mode size is 10 μm, it is advantageous to move the modecenter away from the substrate to avoid substrate loss. The compositewaveguide can be fabricated using any material that will allowillumination of the desired wavelength to propagate therein. Examples ofmaterials from which composite waveguides of the invention can befabricated include crystalline silicon, poly-silicon, amorphous silicon,silicon nitride, silicon oxynitride, silicon dioxide, doped silicondioxide and a polymer.

It is believed that the edge coupler of the invention provides a numberof novel features.

As used herein, the term “directed illumination” means an optical beamthat is observable as a beam travelling in a propagation direction, suchas a beam provided by a laser, a beam focused by a lens, or a beampropagating in an optical carrier such an optical fiber or an opticalwaveguide. In contradistinction, the term “evanescent wave” means thosewaves coupled laterally from a waveguide carrying a directedillumination optical beam into an adjacent waveguide.

The edge coupler provides an apparatus that can couple to large modedirected illumination, which in some embodiments may be >5 μm. Largermodes provide greater tolerance. A 10 μm mode size is standard for fiberand enables packaging that is easier and less expensive to manufacture.As is described in greater detail hereinafter the composite waveguide isconfigured to interact with optical radiation present at the chip facet.In some embodiments, the composite waveguide ends at the facet. In otherembodiments, the composite waveguide ends close enough to a facet toallow light to enter or leave the composite waveguide (e.g., within 10μm, 20 μm, 30 μm, 40 μm, 50 μm, or 100 μm of a facet), but not preciselyat a facet. In the composite waveguide, one waveguide core is tapered tohave a larger facet, and others of the group of waveguide cores taperdown to smaller dimensions. The illumination is concentrated byevanescent waves from the group of waveguide cores into a singlewaveguide which provides an optical output. In some embodiments, thecores of the composite waveguide can be tapered laterally or vertically,and they can taper to a smaller dimension or to a larger dimension.

In other embodiments, one can evanescently couple between different onesof the groups of waveguide cores so long as they are in sufficientlyclose proximity. In other embodiments, one can evanescently between acomposite waveguide having a group of waveguide cores and a differenttype of optical waveguide that is in sufficiently close proximity. Whenevanescent coupling between a composite waveguide and another structure,such as a different type of waveguide, the coupling occurs along asegment of the composite waveguide, and the input port comprises asegment of the composite waveguide. In some embodiments of waveguidesthat perform evanescent coupling, the waveguide does not terminate atthe location where the evanescent coupling is performed. In someembodiments of waveguides that perform evanescent coupling doesterminate at the location where the evanescent coupling is performed. Insome embodiments, the waveguide that performs evanescent coupling canalso include a taper. In some embodiments, the waveguide that performsevanescent coupling turns into a dump port (e.g., the illuminationpassing along the waveguide is lost or discsrede after the location atwhich the evanescent coupling take place). In some embodiments, thewaveguide that performs evanescent coupling can be in the same layer asone of the cores of a composite waveguide, or can be in a differentlayer than one of the cores of a composite waveguide.

In order to prevent optical losses caused by optical coupling into asilicon substrate, the edge couple of the invention is constructed at alevel which is deliberately spaced away from the silicon substrate.

The edge coupler of the invention can also provide one or moretransverse couplers (also termed “vertical couplers”) that are able tocouple optical power in a transverse direction relative to the opticalpath of the edge coupler (e.g., vertically) from the edge coupler to thesilicon device layer. The transverse coupler can be fabricated to allowsequential optical steps through a number of layers using one or morevertical couplers.

The edge coupler of the invention also provides the ability tomanufacture devices comprising the edge coupler using back-endintegration. In general it is difficult to integrate any layers into thechip backend stack. According to principles of the invention, the edgecouplers can be built by using one or more SiN stop layers present inthe CMOS flow that are ordinarily provided for use as chemical/polishetch stop layers.

FIG. 1 is a cross section diagram of a composite waveguide according toprinciples of the invention. In the fabrication line, the metal layersare deposited above the silicon device layer, and the SiN stop layers102, 104 between the metal layers are several microns higher than thesilicon layer, as shown in FIG. 1. Using these SiN layers caneffectively reduce the substrate loss. After the optical power iscoupled to the SiN waveguide core, a vertical coupler can be used totransfer the power to the silicon device layer.

A second feature of this architecture is that the SiN stop layers thatare present in the damascene process can also be used to couple lightvertically in the chip to an output coupler that is further from thesubstrate. This helps move the center of the mode away from thesubstrate, thereby further reducing loss due to substrate coupling.

First Example Embodiment

An embodiment of an edge coupler built using two SiN 102, 104 stoplayers is designed for use with a standard single mode fiber. FIG. 2A isa perspective diagram of a composite waveguide having a group ofwaveguide cores according to the principles of the invention.

The thickness of each SiN layer is 120 nm, and the SiO₂ between the twolayers is 2 μm thick. On each layer, there are two SiN waveguide cores(202, 204) (206, 208) with 2 m center-to-center separation, and thewidth of the waveguide cores are each 300 nm. As illustrated in FIG. 2A,the lower-right waveguide core 208 is used as output, and is taperedfrom 300 nm width×120 nm height to 1 μm width×120 nm height with alength of 100 μm.

In various embodiments, the width of the waveguide core used as thecoupler may be varied, to see how the behavior changes with width. FIG.3A-FIG. 3D are diagrams of mode profiles of the group of waveguide coresfor selected widths of the edge coupler.

FIG. 2B is another perspective diagram of a composite waveguide having agroup of waveguides.

FIG. 2C is a side view diagram showing the optical intensity in thecomposite waveguide of FIG. 2B as a function of distance from the inputfacet. As may be understood from FIG. 2C, at the input end of thecomposite waveguide (the left end of the diagram) the optical intensityin each of the four waveguide cores is approximately equal. At theoutput end of the composite waveguide, the majority of the intensity hasbeen evanescently coupled into one of the waveguide cores (e.g., theoutput waveguide core such as 208 of FIG. 2A), and the intensity isincreased in the output waveguide by a factor of approximately 2 to 3times over the intensity at the input end of the same waveguide core.

FIG. 2D is a schematic diagram of tapered waveguide cores seen in a viewfrom the top. In the embodiment shown in FIG. 2D, there are shown awaveguide core 230 that tapers from a narrower width at the bottom ofthe drawing to a wider width at the top, and a number of waveguide cores240, 250 (and 260 indicated by a dotted arrow, which is below 250 and isoccluded from view, but is the same shape as 250). Waveguide core 240 isnearer the viewer than waveguide core 230 as shown in FIG. 2D, and canbe seen to taper from a width at the bottom of the drawing that is closeto the width of waveguide core 230 to a narrower width near the top ofthe drawing. The arrow labeled “A” at the left of FIG. 2D indicates theorientation of the view shown in FIG. 2E.

FIG. 2E is a schematic diagram of tapered waveguide cores seen in a viewfrom the side. As seen in the embodiment illustrated in FIG. 2E, thewaveguide core 230 is longer than the waveguide core 240, but both havesimilar width dimensions as seen in the side view. Waveguide cores 250and 260 are occluded by waveguide cores 240 and 230, respectively in theview shown in FIG. 2E.

The mode profile of the composite waveguide as shown in FIG. 3A-FIG. 3D,in which the width of the lower-right waveguide core 208 is 300 nm inFIG. 3A, 400 nm in FIG. 3B, 500 nm in FIG. 3C and 800 nm in FIG. 3D. Atthe end of the coupler where all the waveguide cores have equal width,the mode profile is a combination of the modes of all the SiN waveguidecores, and the optical power is equally distributed in the fourwaveguide cores. In order to couple to silicon waveguides, the mode sizemust be compressed. The lower-right waveguide core becomes wider alongthe propagation direction, so that the optical power will beconcentrated in this waveguide core due to mode evolution.

As illustrated in the embodiments shown in FIG. 4A, FIG. 4B and FIG. 4C,the composite waveguide of the invention can have two, three or fourwaveguide cores. In general, the composite waveguide can have a group ofwaveguide cores (e.g., any convenient number N of, wherein in an integerequal to or greater than 2). The geometrical arrangement of the group ofwaveguide cores can be in a linear array or a two dimensional array asseen from an input end or an output end of the composite waveguide. Insome embodiments, the geometrical arrangement of the group of waveguidecores can be a triangular arrangement, a rectangular arrangement, asquare arrangement, a hexagonal arrangement (which can be generated froma group of triangular arrangements), or any convenient geometricalarrangement, such as a circular arrangement. The array can be periodicor non-periodic.

The physics exhibited by the composite waveguide having a group ofwaveguide cores may be explained as follows. The group of waveguidecores interact with each other by way of evanescent electromagneticwaves, so that optical energy or optical intensity can be transferredbetween or among two or more of the group of waveguide cores. Asillustrated in FIG. 3A through FIG. 3D, the optical intensity on a groupof waveguide cores that are in evanescent optical communication cantransfer optical intensity. In FIG. 3A, the evanescent coupling is suchthat significant optical intensity exists at the output end of thecomposite waveguide in each of the four illustrated waveguide cores. InFIG. 3D, the evanescent coupling is such that significant opticalintensity exists at the output end of only one of the four illustratedwaveguide cores in the composite waveguide, and the optical intensity inthe other three of the four illustrated waveguide cores has beendiminished considerably, so that a single one of the waveguide coresserves as the output. In each of the embodiments illustrated in FIG. 3Athrough 3D, the input ends of the four waveguide cores are allsubstantially the same size, and all terminate at a chip facet.

It is believed that evanescent coupling can also be used to cause lightto propagate in and interact with other types of structures such as gainmedia, non-linear media, opto-magnetic media (such as yttrium irongarnet), photoconductive media, and photo-absorptive media.

FIG. 5A is a schematic diagram of an embodiment that illustrates thepropagation of directed illumination along a waveguide, such as acomposite waveguide of the invention. As shown in FIG. 5A, a chip has asilicon device layer 510 and an edge coupler 515 provided therein. Twowaveguides 520 and 540 are present in an aligned configuration, suchthat a propagation direction of illumination in one waveguide is alignedwith a propagation direction of illumination in the other waveguide.Light waves 530 propagate along the waveguides 520, 540 in eitherdirection, and pass from one waveguide to the other in the samedirection as the propagation direction. In some embodiments, thewaveguides 520 and 540 are fabricated from silicon nitride (SiN).

The embodiment of the composite waveguide shown in FIG. 5A can be a buttcoupler that is a continuous extension of the composite waveguide. Inother embodiments, the composite waveguide can provide a discontinuousbutt-coupling between two types of waveguides. The waveguide can be onthe same or different layers of the device.

FIG. 5B is a schematic diagram of an embodiment that illustrates thepropagation of evanescent radiation between two waveguides. As shown inFIG. 5B, a chip has a silicon device layer 550 and an edge coupler 555provided therein. Two waveguides 560 and 570 are present in aconfiguration in which their propagation axes are aligned but they areoffset one from the other in a direction transverse to the propagationdirection, so that a propagation direction of illumination in one isparallel with a propagation direction of illumination in the other butis laterally offset. Light waves 575 propagate along the waveguide 570and light waves 565 propagate along the waveguide 560 in eitherdirection. The light passes from one waveguide to the other byevanescent propagation, as illustrated by light waves 580. Theevanescent propagation can occur in either direction (e.g., fromwaveguide 560 to waveguide 570, or from waveguide 570 to waveguide 560).The light continues to propagate with a waveguide in the same directionas the propagation direction. In some embodiments, the waveguides 560and 570 are fabricated from silicon nitride (SiN).

FIG. 6 is a diagram in cross section of a substrate, such as asemiconductor chip, that includes composite waveguides constructedaccording to principles of the invention. In the embodiment illustrated,the chip has a silicon handle 605, a device layer 610 adjacent thesilicon handle, and a group of SiN etch stop layers 620, some of whichare used to define and construct a series of metal interconnect layerssuch as copper (Cu) layers, and electrical terminals constructed fromaluminum (Al). The edge connector is illustrated at the upper right sideof the chip and is constructed of waveguide cores 630 fabricated fromsome of the SiN layers. Illumination can enter the chip from a facet atthe right side. In some embodiments, the substrate can be a support thatis not a semiconductor chip.

FIG. 7 is a diagram in cross section of another view of thesemiconductor chip of FIG. 6, in which the chip facet, the edge couple,the vertical couplers, the multilevel metallization, the device layerand the silicon handle are illustrated. In particular in FIG. 7, thereare present a number of SiN stop layers 720 that are provided as part ofa CMOS fabrication process. The SiN layers can be used as elements ofvertical SiN couplers 710 that operate evanescently as illustrated inthe embodiment shown in FIG. 5B.

Embodiments of the coupler can be used to couple in light from anexternal source such as a single mode fiber, an external laser, oranother chip. Embodiments of the coupler can be used to receive lightfrom an on-chip source such as an on-chip laser. Embodiments of thecoupler may also be used to couple light from an on-chip laser out to anexternal target. Embodiments of the coupler may also be used to couplelight in both directions simultaneously. Embodiments of the coupler mayalso be used to couple bidirectionally. The term “bidirection coupling”can mean in two directions at the same time (also described as duplexoperation), or the term “bidirection coupling” can mean coupling in twodirections at distinct times (also described as half-duplex operation).

FIG. 8 is a diagram 800 in cross section of another view of thesemiconductor chip of FIG. 6. In FIG. 8 there is shown in dotted outlinea mode field 830 of illumination that is provided by a source, such asan optical fiber connected to a laser. The mode field is larger in sizethan the dimensions of the composite waveguide, or edge coupler. Themode may extend above the silicon chip surface. There are three optionsthat may be employed to deal with this circumstance. In one embodimentthere is an option to do nothing and let the mode interact with air.This is illustrated in FIG. 7. In one embodiment there is another optionto provide additional silicon dioxide above the chip so that the mode ismore fully enclosed. This may be viewed in FIG. 8 as adding the indexmatching material 820. In one embodiment there is a third option to addan index matching material other than oxide such as index-matching gelor index matching epoxy to the top of the chip. This is an example ofproviding a different index matching material 820.

In addition to the index matching gel or epoxy on the top of the chip,it may be advantageous to provide the same type of material at the edgefacet of the chip when coupling to a fiber or another chip. This isillustrated as interface to fiber or other chip 810. This same materialcan then be deposited just once to be present at both the edge interfaceand the vertical surface (near the edge). A material with structuralproperties such as epoxy can then be used for the purposes of bonding afiber (or second chip) to the chip, providing an index-matched interfaceat the edge of the chip and providing an index-matched cladding abovethe silicon chip surface.

In some embodiments, the composite waveguide comprises a group ofwaveguide cores to couple to fibers. In some embodiments, the group ofwaveguide cores are arranged in one or two dimensions.

In some embodiments, the interconnect process in the back end usesnitride in the back end that is left over from the damascene processesused to build the back-end metal stack.

In some embodiments, one can explicitly add nitride layers which are notleft over from the back end metallization process. In some embodiments,the nitride layers are interspersed into the back end metal stack.

In some embodiments, the waveguide cores have tapers with varyinggeometries.

In some embodiments, there is provided independent phase controls ondifferent waveguide cores that can be used for beam steering.

In some embodiments, photodetectors are integrated into the compositewaveguide structure. The photodetectors can be used for monitoring theoptical signals that propagate in the waveguide cores.

In some embodiments, a waveguide core comprises an etched facet.

In some embodiments, a waveguide core comprises a polished facet.

In some embodiments, a waveguide core comprises a sawed facet.

In some embodiments, a waveguide core comprises a facet oriented at anangle to the length dimension of a waveguide core.

In some embodiments, an angled facet is used to change a plane ofpropagation of illumination.

In some embodiments, a lens is provided between an end of a waveguidecore and an end of an optical fiber. The lens in some embodiments may beoutside of the chip that carries the waveguide core.

In some embodiments, the composite waveguide is used in conjunction witha tapered fiber.

In some embodiments, the composite waveguide is used in conjunction witha small mode fiber.

In some embodiments, the devices of the invention are used to manipulatepolarizations of propagating light, with manipulation of one or both oftwo different polarizations.

In some embodiments, the edge couplers of the invention are used tocouple light into or out of a high confinement silicon waveguide.

In some embodiments, the edge couplers of the invention are used tocouple light to another optical carrier through free space, for exampleusing external lenses.

In some embodiments, there are provided integrated structures that usethe low confinement of the waveguide. For example, a grating can beprovided to take advantage of the low confinement.

In some embodiments, the edge couplers of the invention are provided incombination with semiconductor optical amplifiers (SOAs) or other gainmedia to provide gain. It is believed that one can thereby provide gainwithout having to etch the oxide.

In some embodiments, nonlinear optical media can be provided inconjunction with the edge couplers of the invention, for example bybeing bonded to a surface of a wafer.

Design and Fabrication

Methods of designing and fabricating devices having elements similar tothose described herein are described in one or more of U.S. Pat. Nos.7,200,308, 7,339,724, 7,424,192, 7,480,434, 7,643,714, 7,760,970,7,894,696, 8,031,985, 8,067,724, 8,098,965, 8,203,115, 8,237,102,8,258,476, 8,270,778, 8,280,211, 8,311,374, 8,340,486, 8,380,016,8,390,922, 8,798,406, and 8,818,141, each of which documents is herebyincorporated by reference herein in its entirety.

Definitions

As used herein, the term “optical communication channel” is intended todenote a single optical channel, such as light that can carryinformation using a specific carrier wavelength in a wavelength divisionmultiplexed (WDM) system.

As used herein, the term “optical carrier” is intended to denote amedium or a structure through which any number of optical signalsincluding WDM signals can propagate, which by way of example can includegases such as air, a void such as a vacuum or extraterrestrial space,and structures such as optical fibers and optical waveguides.

As used herein, the term “damascene” or “damascene process” refers to aprocess for forming copper IC interconnects using an additive processingtechnique. The Damascene process was originated by IBM in the 1990s, andhas been described in numerous patent documents, including U.S. Pat.Nos. 5,262,354, 5,300,813, 5,403,779, 5,426,330, 6,140,226 and manyothers.

The Damascene process includes depositing a dielectric on asemiconductor wafer, such as an SOI wafer; etching the dielectricaccording to a defined photoresist pattern; depositing a barrier layer;depositing a conductor such as copper, for example by electroplating, insome cases using a two-step process in which a seed layer is depositedon the wafer using PVD, and then electroplating copper is electroplated,followed by a planarization, for example by CMP. In some embodiments,the dielectric is silicon nitride. In other embodiments, the dielectriccan be made of other materials, such crystalline silicon, poly-silicon,amorphous silicon, silicon nitride, silicon oxynitride, silicon dioxide,doped silicon dioxide and polymer.

A Dual Damascene process can also be used, for example to create viasand lines by etching holes and trenches in the dielectric, and thendepositing copper in both features. One photo/etch step is used to makeholes (vias) in the dielectric so as to make connection with underlyingmetal, while a second photo/etch step is used to make trenches for themetal line. The two photo/etch steps can be performed in either order.

As used herein, the term “front end of the line” refers to the portionof a semiconductor fabrication process or facility in which a waferhaving individual components or devices (e.g., transistors, capacitors,resistors, inductors, and the like) are fabricated on a wafer.

As used herein, the term “back end of the line” refers to the portion ofa semiconductor fabrication process or facility in which the individualcomponents or devices are interconnected with metallization (or wiring)on the wafer. The metallization can include multiple metallizationlayers.

As used herein, the term “optical propagation direction” as applied to awaveguide having at least two ends means that an optical signalpropagates along the waveguide including going around bends if thewaveguide is so configured, so that the optical signal enters thewaveguide at a first location, such as a first end, and exists thewaveguide at a different location, such as a second end. For a closedloop waveguide, the propagation direction is around the loop in a firstsense or in a second sense, e.g., clockwise or counterclockwise.

THEORETICAL DISCUSSION

Although the theoretical description given herein is thought to becorrect, the operation of the devices described and claimed herein doesnot depend upon the accuracy or validity of the theoretical description.That is, later theoretical developments that may explain the observedresults on a basis different from the theory presented herein will notdetract from the inventions described herein.

Any patent, patent application, patent application publication, journalarticle, book, published paper, or other publicly available materialidentified in the specification is hereby incorporated by referenceherein in its entirety. Any material, or portion thereof, that is saidto be incorporated by reference herein, but which conflicts withexisting definitions, statements, or other disclosure materialexplicitly set forth herein is only incorporated to the extent that noconflict arises between that incorporated material and the presentdisclosure material. In the event of a conflict, the conflict is to beresolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and describedwith reference to the preferred mode as illustrated in the drawing, itwill be understood by one skilled in the art that various changes indetail may be affected therein without departing from the spirit andscope of the invention as defined by the claims.

What is claimed is:
 1. A photonic integrated circuit (PIC) comprising: asubstrate; an optical device layer supported over the substrateincluding a first waveguide; and an edge coupler for coupling lightbetween the first waveguide and a second waveguide, which includes alarger mode size than the first waveguide comprising: a compositewaveguide including a plurality of spaced apart waveguide cores capableof evanescent coupling therebetween supported over the substrate, outerends of which extend proximate an edge of the PIC for optically couplingto the second waveguide enabling the transfer of the light between thesecond waveguide and the plurality of waveguide cores, and inner ends ofthe waveguide cores extend into the PIC; and an optical input/output atthe inner end of a first waveguide core of the plurality of waveguidecores capable of transferring light between the first waveguide core andthe first waveguide.
 2. The PIC according to claim 1, wherein the firstwaveguide core includes an end section that expands towards the innerend thereof for increasing evanescent transfer of the light between theother of the plurality of waveguide cores and the first waveguide core.3. The PIC according to claim 2, wherein the first waveguide core islonger than the other of the plurality of waveguide cores.
 4. The PICaccording to claim 1, wherein each of the plurality of waveguide coresother than the first waveguide core tapers down towards the inner endthereof.
 5. The PIC according to claim 1, wherein the first waveguidecore is vertically spaced between the substrate and a second one of theplurality of waveguide cores, whereby the light travels towards or awayfrom the substrate.
 6. The PIC according to claim 5, wherein the firstwaveguide core is laterally spaced from a third one of the plurality ofwaveguide cores.
 7. The PIC according to claim 6, wherein first andthird waveguide cores are provided in a first stop layer of dielectricmaterial; wherein the second waveguide core is provided in a second stoplayer of dielectric material; and further comprising metallizationbetween the first and second stop layers of dielectric material forelectrically communicating with the optical device layer.
 8. The PICaccording to claim 1, wherein the optical input/output is capable ofevanescently transferring the light between the first waveguide and thecomposite waveguide.
 9. The PIC according to claim 1, wherein theoptical device layer is between the composite waveguide and thesubstrate.
 10. The PIC according to claim 1, further comprising indexmatching material on an upper surface of the PIC for enclosing thelarger mode from the second waveguide.
 11. The PIC according to claim 1,wherein the optical device layer includes a laser for generating thelight for output the composite waveguide to the second waveguide. 12.The PIC according to claim 1, wherein the optical device layer includesa photodetector for receiving the light from the second waveguide viathe composite waveguide.
 13. The PIC according to claim 1, wherein thecomposite waveguide is capable of converting the mode size of the lightbetween less than 1 μm and greater than 5 μm.
 14. The PIC according toclaim 1, wherein the composite waveguide is capable of converting themode size of the light between less than 1 μm and about 10 μm.
 15. ThePIC according to claim 1, wherein the outer end of each waveguide coreincludes a width of less than 1 micron and a thickness of less than 1micron.
 16. The PIC according to claim 1, wherein each waveguide coreincludes a thickness of about 120 nm.
 17. The PIC according to claim 1,wherein the end section of the first waveguide core includes a widththat expands from about 300 nm to about 1 μm.
 18. The PIC according toclaim 1, wherein each waveguide core comprises a dielectric orsemiconductor material enabling illumination of a desired wavelength;and wherein the semiconductor or dielectric material is selected fromthe group consisting of crystalline silicon, poly-silicon, amorphoussilicon, silicon nitride, silicon oxynitride, silicon dioxide, dopedsilicon dioxide, and a polymer.
 19. A method of manufacturing a photonicintegrated circuit (PIC) comprising: providing an optical device layerincluding a first waveguide on a substrate; depositing a first stoplayer; depositing a metal layer; depositing a second stop layer; andpatterning the first and second stop layers into a plurality ofwaveguide cores forming an edge coupler for coupling light between thefirst waveguide and a second waveguide with a larger mode size than thefirst waveguide, the edge coupler comprising: a composite waveguideincluding the plurality of spaced apart waveguide cores capable ofevanescent coupling therebetween, outer ends of which extends to an edgeof the PIC for optically coupling to the second waveguide enabling thetransfer of light between the second waveguide and the plurality ofwaveguide cores, and inner ends of which extend into the PIC; and anoptical input/output at the inner end of a first waveguide core of theplurality of waveguide cores for transferring the light between thefirst waveguide and the composite waveguide.
 20. The method according toclaim 19, wherein a first waveguide core of the plurality of waveguidecores includes an end section that expands towards the inner end thereoffor increasing evanescent transfer of light between the other of theplurality of waveguide cores and the first one of the plurality ofwaveguide cores.